Trouble With 2000 yz426f!

yamaha426riderr

I was out riding my bike recently when I was in 4th gear about mid throttle and it killed on me.I started it again an got back down on it an it did the same thing.Afterwards,the bike didn't want to start at all an it felt as if it had lost compression.new spark plug still won't start..could this be the valves.??I hope this is an easy fix..